1. Field of the Invention
The present invention relates to a sheet feeding device and an image forming apparatus and more specifically to a device for loosening a sheet stack with air before each sheet is fed.
2. Description of the Related Art
Conventionally, an image forming apparatus such as a copying machine, electrophotographic printer, ink jet printer, facsimile, a printing machine is equipped with a sheet feeding device for feeding the sheets one by one from a storage portion in which a plurality of the sheets are stored.
As such a sheet feeding device, recently, there has been proposed a sheet feeding device using air suction method of conveying a sheet by sucking the sheet loaded on a tray, this sheet feeding device using air suction force and conveying force of an endless belt. This technology has been described in Japanese Patent Application Laid-Open (JP-A) Nos. 07-89625 and 2005-104723. In the meantime, such a sheet feeding device has been often used in the image forming apparatus having a high productivity and for which a replacement component having a long service life is demanded.
As shown in FIG. 14, such a conventional sheet feeding device has a storage portion 100 having a tray 101 capable of rising/lowering and a suction conveying belt 126 which conveys a topmost sheet S1 of the sheets S stacked on the tray 101 by suction. This suction conveying belt 126 is capable of conveying the topmost sheet S1 of the stacked sheets when it is located at a predetermined height (feeding enabled range). Then, a paper lower limit detection sensor 122 for detecting that the topmost sheet S1 of the stacked sheets reaches the lower limit position of the feeding enabled range and a paper face upper limit detection sensor 123 for detecting that the topmost sheet S1 reaches the upper limit position of the feeding enabled range are provided.
When the sheet is conveyed in the conventional sheet feeding device having such a structure, the sheet S is loaded on the tray 101 which is provided within the storage portion 100 and can be lifted up/down by a driving means (not shown) and then the tray 101 is lifted up/down. Next, the topmost sheet S1 presses a paper face detection lever 121 and this paper detection lever 121 is detected by the paper face lower limit detection sensor 122 (turns ON) so as to stop the tray 101.
Next, air supplied from a fan (not shown) is blown to an end portion of a sheet stack through a loosening duct 151 to float several pieces of the sheets on the top portion of the sheet stack into a loosened state. At this time, the paper face detection lever 121 is pushed up by the topmost sheet S1 floated and then, this paper face detection lever 121 is detected by the paper face upper limit detection sensor 123 (turns ON). When the tray 101 is lowered, detection of the paper face detection lever 121 by the paper face upper limit detection sensor 123 is eliminated (turns OFF) and then the tray 101 is stopped.
By controlling rising and lowering of the tray 101 in this way, the topmost sheet S1 can be maintained between the upper limit position and the lower limit position and when the topmost sheet S1 is located between the upper limit position and the lower limit position, the control device in the main body determines that feeding of the sheets is enabled.
After the top face position of the topmost sheet S1 comes into an appropriate range enabling it to be fed, sheet feeding operation is carried out. When the sheet feeding operation is started, first, the topmost sheet S1 is sucked to the suction conveying belt 126 by the suction fan 125 provided within a sheet feeding portion 12. Next, the suction conveying belt 126 is rotated by a driving means (not shown) so as to feed only the topmost sheet S1 in a direction of an arrow indicated in FIG. 14 and then, the sheet S1 fed in this way is conveyed to a downstream side by conveying roller pair 161.
If the sheet S on the tray 101 decreases due to successive execution of such sheet feeding operation, the position of the paper face drops accompanied thereby, so that detection of the paper face detection lever 121 by the paper face lower limit detection sensor 122 is eliminated (turns OFF). Then, in such a case, the tray 101 is raised to position the top face position of the topmost sheet S1 within a feeding enabled range.
However, the conventional sheet feeding device cannot determine whether or not a second and following sheets are loosened (in a floating condition) because the paper face upper limit detection sensor 123 can detect only the top face position of the topmost sheet.
For example, in an action before feeding a sheet in order to position the topmost sheet S1 in an appropriate range, sometimes, only the topmost sheet S1 is floated by air flown to the sheet front end portion. At this time, the paper face detection lever 121 is rotated by the topmost sheet S1 so that it is detected by the paper face upper limit detection sensor 123 and then the tray 101 is lowered. In this condition, the topmost sheet S1 is tilted such that its rear end side is lowered as shown in FIG. 15 because the second and following sheets are not floated.
However, in this condition, the front end side of the topmost sheet S1 is floated and the paper face upper limit detection sensor 123 detects the paper face detection lever 121 and consequently, the tray 101 continues to be lowered. Thus, the paper face upper limit detection sensor 123 does not detect the paper face detection lever 121 and when the tray 101 is stopped, the topmost sheet S1 is stopped with a large tilting condition.
As a result, the upstream portion in the sheet feeding direction of the topmost sheet S1 becomes far from the suction conveying belt 126 largely, so that sometimes, suction of the sheet by the suction conveying belt 126 cannot be executed. In this case, sheet feeding failure occurs, so that jamming is generated.
Even if the capacity of the suction fan 125 is so high that the topmost sheet S1 can be sucked and conveyed, it takes time to lift up the tray 101 up to the sheet feeding lower limit position in which the paper face lower limit detection sensor 122 detects the sheet because a next sheet droops largely. If it takes time for the tray 101 to be lifted up, feeding of the sheet becomes slow thereby lowering productivity. Further, if the next sheet delays, it is detected that the sheets are not conveyed at an appropriate interval by a sensor or the like, and consequently, the sensor determines that it is jamming by mistake.
Although in the sheet feeding device mentioned in the JP-A No. 07-89625, a position (height) of a sheet on the front end side is measured with a distance measuring sensor and the discharge amount of loosening air is controlled by a fan rotation number so that the topmost sheet is located at an appropriate position, this structure can only detect the position of the topmost sheet. Therefore, loosening condition of the sheets cannot be determined.
On the other hand, in the sheet feeding device of the above described JP-A No. 2005-104723, the sheet is photographed with an image pickup element such as CCD disposed on a sideway of the sheet stack and image processing is carried out to determine a loosening condition of the sheets located under the topmost sheet. However, in this case, the device is complicated and cost is increased.